Some third-party services ask you to add a short verification token or code to your RSS feed to confirm you own the show. ART19 doesn't have a dedicated field for these tokens, but anything on your series that appears in the feed will do the job.
The tidiest place is the Tags field, in the Information card on your Series Details page. Tags appear in your RSS feed but aren't shown anywhere public-facing, so your verification code stays out of sight while still being readable by the service. You can also add it to your series description if you prefer.
Once it's saved and live on your feed, go back to the service and click Verify. Adding the token won't disrupt your feed - it's just text - and you can remove it afterwards if you like.